1. Field of the Invention
The present invention relates to an electronic appliance of any kind, and also relates to a semiconductor integrated circuit device provided with a power control function for turning on and off the power to such an electronic appliance. More particularly, the present invention relates to an electronic appliance such as a DSC (digital still camera) and a cellular phone, and also relates to a semiconductor integrated circuit device designed for use in such an electronic appliance and provided with a power control function for turning on and off the power to the electronic appliance.
2. Description of the Prior Art
Conventionally, many electronic appliances such as DSCs and cellular phones operate by being supplied with electric power from a direct-current power source. Such an electronic appliance that operates from a direct-current source is supplied with electric power from a battery, which may employ a secondary cell that is rechargeable for repeated use. Accordingly, there have been proposed many methods for reducing the power consumption in an electronic appliance for the purpose of prolonging the life of the battery that supplies the electronic appliance with electric power.
One conventionally proposed method relates to a receiver that, once brought into synchronism with a transmitter for communication, performs communication operations only during periods allocated to the receiver itself. This receiver operates by using a high-frequency clock signal only during the periods in which it performs communication operations, and operates by using a low-frequency clock signal in the periods in which it does not perform communication operations. This helps reduce power consumption. Another conventionally proposed method relates to a communication terminal that is so configured that, even when the power thereto is on, a low-rate clock signal is used and no clock signal is fed to the central processing unit except during reception.
In a communication appliance such as a cellular phone that performs communication operations only during the periods allocated thereto, the methods described above help reduce power consumption by keeping the central processing unit in a sleeping state during the periods in which no communication operations are performed and therefore the central processing unit does not need to operate. However, this is possible only in cases where, as described above, a communication apparatus operates in synchronism with another communication appliance and the periods in which no communication operations are performed can be accurately predicted. Accordingly, these methods are not used in electronic appliances such as DSCs, and, even if used, they offer no advantages.
In an electronic appliance as mentioned above, when the main power starts to be supplied thereto, to prevent failure, it is necessary that the microcomputer for controlling the individual blocks in the electronic appliance be first completely started up and then brought into an active state in which it can perform control operations. To achieve this, conventionally, the supply voltage is constantly monitored to check whether or not the main power is on so that, when the main power is turned on, the microcomputer is brought into the active state. Here, if the microcomputer needs to operate constantly to check whether or not the main power is on, it consumes power even when the main power is off, resulting in increased power consumption.
This can be overcome by bringing the microcomputer into the steady, active state only immediately after the main power is turned on, and this can be achieved by providing, as shown in FIG. 4, a voltage detection circuit 101 that checks whether or not the main power is on and a delay circuit 102 that adds a delay to the detection signal from the voltage detection circuit 101 to output a control signal for bringing a microcomputer 103 into an active state. Here, thanks to the voltage detection circuit 101 and the delay circuit 102, in a stand-by state with the main power off, only the voltage detection circuit 101 and the delay circuit 102 need to be kept on with the microcomputer 103 left off.
When the main power is turned on, the microcomputer 103 starts to be supplied with the main supply voltage and is thereby turned on. At this time, the voltage detection circuit 101 detects the main supply voltage and outputs a detection signal, and the delay circuit 102 adds a delay thereto and feeds it as a control signal to the microcomputer 103. In this way, the delay circuit 102 produces a delay long enough for the microcomputer 103 to start up completely after the supply voltage starts to be supplied thereto. This helps protect the microcomputer 103, and also helps reduce the power consumption in the stand-by state.
Inconveniently, however, an electronic appliance of which the starting-up is controlled as shown in FIG. 4 has the following disadvantages. To keep the voltage detection circuit 101 and the delay circuit 102 operating when the microcomputer 103 is not, the voltage detection circuit 101 and the delay circuit 102 need to be configured not as digital circuits but as analog circuits, which require a larger number of circuit elements and a larger mounting area (it should be noted that, in FIG. 4, the blocks corresponding to those identified with numerals 2a, 2b, and 4 to 7 in FIG. 1 are omitted). Moreover, the delay circuit 102 includes a capacitor, to which an increasingly high capacitance needs to be given as an increasingly long delay is required. This further increases the mounting area. That is, configuring the voltage detection circuit 101 and the delay circuit 102 as analog circuits helps reduce the power consumption in a stand-by state before the power is turned on, but only at the cost of increasing the size of the device